The invention relates to a novel screw tensioning device with a screw tensioning cylinder with a high-pressure hydraulic pump for pretensioning screws and loosening extremely secure pretensable screw connections and a method for controlling this screw tensioning device.
In addition to the basic forms of various screw connections, so-called extremely secure pretensable screw connections have already been used for decades, above all in steel construction. Various screw tensioning devices exist for that. These screw tensioning devices all operate according to the same well-known principle as a rule based on torque-free pretensioning methods. A screw tensioning cylinder supported on the component or machine part that is to be tensioned is screwed onto the thread end over the nut of a threaded bolt to be tensioned with a movable working piston or a threaded bushing located in the working piston here. After that, a high-pressure hydraulic pump is connected to the screw tensioning cylinder that pressurizes the piston-ring side and consequently brings about a retraction of the working piston and the pretensioning of the threaded bolt. The threaded bolt retains its pretensioned force and tensions the machine part after the pressure is released on the piston-ring side because of the turning of the nut onto the machine part as far as it will go. Hydraulic screw tensioning devices with a multi-stage design are described, for example, in DE 10 2004 043 145 B3, in DE 196 38 901 A1 and in GB 22 91 155 A. The single-stage design is described in detail in EP 0 200 459 A2, for instance. A method for connecting or fastening components using an extremely secure pretensable connection is achieved in EP 2 014 932 A2 that is supposed to make especially favorable controlled pretensioning possible. In so doing, a special dimensioning of the projecting screw shaft and the nut with the aid of an oversized washer is proposed. A unit based on a hydraulically actuated screw tensioning cylinder in the form of a screw tensioning device is also discussed in this technical solution. This design is only suitable for certain specially pretensioned screw connections, though, and it is not universally applicable.
The advantages of the extremely secure pretensioning method vis-a-vis the tightening of the screw connection by solely applying a torque to the nut basically involve the fact that the threaded bolt is not additionally stressed with torsion during the tightening process and that no frictional forces arise in the threading and on the nut support. It is possible because of this to realize considerably higher pretensioning forces in the screw connection vis-a-vis a normal tightening of the nut and to effectively increases its load-bearing capability. Very large variations in the frictional forces especially arise, based on experience, in large thread diameters over 30 mm, which is why the load-bearing capability is significantly reduced when the nut is tightened by means of a pure torque.
In addition to the advantages that were described, the hydraulic screw tensioning devices in the prior art have a significant drawback in that additional, cost-intensive high-pressure hydraulic pumps are required to operate the devices. High-pressure hydraulic lines, frequently even very long hydraulic lines on top of that, are likewise required to connection the pump and the screw tensioning device. The bulky hydraulic lines especially lead to problems and to time-consuming preparatory work when there are cramped space circumstances, for instance in the gondola of a wind energy system where systems of this type are frequently used. The lines can only be laid with relatively large radii, because they are not permitted to be crimped, resulting in large space requirements.
Several extremely secure screw connections have to frequently also be tightened with the same number of screw tensioning devices, which is why substantial expenses can arise in the preparation of the hydraulic lines and in their installation on site. Moreover, the pump has to also be transported to the respective assembly location and appropriate supplied with sufficient energy there. The problem of additional equipment becomes particularly clear when there is work under water, as frequently arises in the offshore industry, for instance. Very long high-pressure lines from the pump are required from a ship or other platform out to the assembly location under water here. Hydraulic fluids are also damaging to the environment as a rule and therefore require a series of time-consuming, cost-intensive measures for certain applications.
There are also applications where the use of hydraulic liquids is only approved under strict safety conditions in general.